I think it may be due me associating the vented rib to the Python. It does, however, look better to me with the 6" barrel. I have owned a few MK IIIs over the years and all of them had the solid rib. I may be crazy, but while I like the MK III with a 4", I do not prefer the Python with a 4" barrel and given the opportunity to obtain one, would only consider the 6" barrel.
A letter won't state a MKIII has a vented barrel because it is still a MKIII and was a transition gun with no record kept on how many vented barrels were attached to the MKIII. These were made in 1983. Here's my MKIII with a vented rib and a letter from Colt post marked Feb 1st 1984.